In these days, research and development on various light-emitting elements have been vigorously conducted. Of them, organic electroluminescent (EL) elements have attracted attention as promising light-emitting elements due to their advantages of ultra-thin thickness and light weight, high response speed, wide viewing angle, and low driving voltage. In general, the organic EL elements comprise a light-emitting layer sandwiched by a pair of opposed electrodes and utilize a light emitted from excitons generated by recombination of an electron injected from a cathode and a hole injected from an anode.
Presently known organic EL elements capable of giving a highly bright luminance at a low voltage are those which are disclosed by Tang and which have a layered structure (Applied Physics Letters, vol.51, p.913, 1987). These elements have an electron-transporting and light-emitting material and a hole transporting material in a layered form which serves to emit a high-luminance green light, and they give a luminance as high as several thousands cd/m2 at a direct current voltage of 6 to 7 V. However, in consideration of their use as a full color display or a light source, emission of lights of three primary colors or a white light is practically necessary. The above-described elements use a 8-quinolinol aluminum complex as a light-emitting material, and the color of the light emitted therefrom is limited to green. Hence, light-emitting elements capable of emitting a light of other color have been desired to be developed. As materials capable of emitting a light of a longer wavelength than green, there have so far been developed such light-emitting materials as 4-(dicyanomethylene)-2-methyl-(4-dimethylaminostyryl)-4H-pyran (DCM) and its derivatives, Nile Red derivatives and Eu (III) complexes. However, they have the defects of, for example, poor color purity, low luminance of an emitted light, low light-emitting efficiency, and low durability.
Thus, various investigations have been made. Elements using compounds having a cyclic acidic nucleus such as those compounds which are described in JP-A-11-335661 were prepared and evaluated and were found to be excellent in red color purity, emitted light luminance, light-emitting efficiency and durability.
However, in the case of preparing an element wherein one and the same compound is used as the electron injecting and transporting layer on the cathode side and a host material in the light-emitting layer as with the vacuum deposition type element described in the Example, there result a poor luminance response and a large temperature dependence of the pulse width of an emitted light in pulse driving. Therefore, in the case of applying them to light sources for exposure, they are not adapted for creating a wide gradation by pulse modulation. Thus, improvement of response properties upon pulse driving, particularly temperature dependence of the pulse width of an emitted light, have been desired. Also, light-emitting efficiency has been desired to be more improved.